Lack of practice and like Chinmoy said, mental block! Also the fear of being ridiculed when you speak something in the wrong manner! is what deters most people from speaking English fluently!

yes , lack of practice of speaking, If they practicing daily then they can speak English fluently

I also observe that there are many professionals in chartered accountancy, medicine and law who are proficient in English but cannot converse fluently in English.

We need to differentiate between spoken and written English. Proficiency in written language does not give you skill in spoken language. We need to practice reading and writing for written language. But spoken language needs listening and speaking. It is necessary to see English language movies, TV shows, radio broadcasts and speak in English preferably with individuals expert in spoken English.

Personally, I did not know English beyond simple words and alphabets till class V. On my fathers's posting to Kanpur, I was admitted in a school that imparted English language study from primary level. This caused me a feeling of inferiority. However, I began speaking, talking, reading and writing in English only. Within six months, I was at the top in my class. In this connection, please see my article. The link is given below:

http://www.writing.com/main/view_item/item_id/1502510-my-way-of-learning

well explained by Gulshanji, thank you for the link and explaination

well explained by Gulshanji, thank you for the link and explaination

Let me correct your spelling. 'Explain' is correct but 'explaination' is wrong. This should be 'explanation'.

You need to start communicating in english with the person,who knows english very well. This will help to come out different words which is in our mind,but not coming out. I mean the knowledge will improve.

To get the speaking of the english fluently, we need to have great practice of english, as much speaking practice we have that much fluent we can speak in english. for have good practice we should be in between the people who know the language which you know. the communication between you must be english only then only your english develops professional level

I remember when i was in abroad in Nigeria,i was with the people who only know one language,that is english,so i was only having one choice and that to communicate in english. There i have improved a lot and from there i got understood that English language is very important and you should know this language.

Yes english is a universal language..if one can speak english ,im sure that he will find someone to talk to in every country of the world.

Speaking in english is really a good thing. to improve it we need to speak and read alot